1. Field of the Invention
The present invention relates to a telephone switching system, and more particularly to an electronic telephone switching system capable of automatically transmitting, when a calling subscriber encounters any destination that is busy, a voice message indicating the length of time from the beginning of that busy status till the current point of time, i.e. the duration of the busy status, instead of a busy tone to the calling subscriber.
2. Description of the Prior Art
A telephone switching system, upon receiving dial number information from a calling subscriber, identifies the telephone number of the destination, sends out a ringing signal to the telephone set of the destination, and sends back a ring back tone to the calling subscriber. When the subscriber at the destination takes up the handset of his telephone, the switching system detects the off-hook status, stops sending the ringing signal and the ring back tone, and operates a communication path switch to connect, and to keep connected, the calling and the called subscribers.
Meanwhile, if the called subscriber is communicating with another subscriber when a call is initiated by said calling subscriber, the switching system will send out a busy tone to the calling subscriber. The calling subscriber knows by this busy tone that the destination of his call is busy, and puts his handset on the hook, in response to which the switching system is released.
In order for the calling subscriber to achieve connection with his desired destination, he should repeat the call initiating procedure some time after he puts his handset on the hook. If the destination is still busy, he should repeat the procedure until the connection is finally achieved.
To dispense with this troublesome repetition of the call initiating procedure, an electronic messaging service is proposed in the U.S. Pat. No. 4,932,042, "Spontaneous Voice and Data Messaging". The service thereby provided is to send an announcement signal regarding the availability of a messaging service, either simultaneously with or instead of a busy tone, to the calling subscriber. A calling subscriber wishing to use this messaging service dials a prescribed three-digit number. Upon detection of this three-digit number by the switching system, the calling subscriber is connected to a voice message operation center (VMOC), which is supplied by the switching system with the telephone numbers of the calling and called subscribers and billing information.
Meanwhile, in order to verify the telephone number of the destination of the message, the VMOC sends an announcement signal to the calling subscriber. As the calling subscriber responds to that signal by operating a dial key, the VMOC receives and records a voice message from the calling subscriber. Upon completion of the reception and recording of this voice message, call initiation to convey that message is accomplished by the prior art.
The use of this messaging service would require the payment of a subscription fee for this service in addition to the usual fixed monthly due for subscription to the telephone system, because the installation and operation of said VMOC costs a large sum of money. The collection of this additional fee is preventing the messaging service according to said patented invention from being used more extensively.